It may just be BFBC2 or any prolonged high gfx application, but it currently only has happened on BFBC2, and not Napoleon, AC2 or 3DMark.

Basically after half to one hour of play the computer crashes with a blank screen (no signal) and both LEDs light up on the gfx card, the only thing to do is shutdown and boot up, sometims it takes up to a minute to POST and the LEDs stay on.

I have actually had this problem before, the first time was solved by a new PSU, the second by cleaning dust off the heat sink, the third by taking the side of the case off, I am confident it is not overheating anymore, but I suppose it is possible that my new PSU is a dud (though its Antec)

Looking at HWMonitor graphs of voltage the only one that falls below the correct value slightly during intense load is +3.33V. Does that matter?

yeah sounds like the card is thermal throttling for some reason thats why the lights stay on upon bootup. whaich heatsink revision is it?? do you run the fan at 100%?? also try pointing another 120mm fan on it while gaming just as an experimental basis. whats the temps?? and did you take the heatsink off and re apply thermal paste??

How many watts is your PSU? On your profile it says 650 watts, which should be fine. However, if you are running Crossfire, then it be pushing it a little since 4xxx series consume more power than the 5xxx series

sooooooo here is your Poblem(s)
1: that heatsink is good yes but it does not cover your voltage regulators AT ALL they are probably overheating slap a 1200 fan over that bitch
2: your gpu core is throttling 90c load is at the throttle point.

take the heatsink off re apply thermal paste use the 120mm fan. My best personal advice is buy an aftermarket heatsink. but if you can afford a good one, like a thermalright trad2 or something like that then try the 120mm fan pointed in front of it closer to the powerconnectors this way it blows on the vregs and helps move some of the gpu core air as well.

I put some artic cooling paste on the gpu, gave it a good old dust, removed the annoying plastic thing on the heatsink blocking air flow, and put a 60mm fan pointing at the right hand side of the card. Nice drop in temperature.

After doing even more research, I saw some guy on the internet mention that the problem may be caused due to the system thinking you have more VRAM than you do, mine thinks I have 2GB when it actually has 512MB.

Another thing is a BIOS update on the ASUS site (which I can't run) says: "Fix game may be stopped automatically due to graphics card is overheat."

That ASUS BIOS update won't be for your card. It was for a very specific limiter batch of parts with the Part/Number: of *-C3CGG0-* & *-C3CGK0-*. These numbers will be on the box your card came with. If it doesn't flash it, it's not your card.

I had a look at your BIOS. Is that the original BIOS your card shipped with?

what are you trying to run the bios update with?? are you using atiflah or atiWINFLASH?? with atiflash you can force it to flash whereas winflash you cannot. Also dayum that did cause a huge temp drop man!! 30c is one hell of a drop. now what do you mean by it thinks it has to much ram?? on the gpu?? can you show me where it says 2gb??

My 5850 throttles when it reaches over 75 deg and starts skipping and doing short freezes. when i play games i run my fan profile on 50% and it doesnt get over 62 deg full load. i suggest a better cooler on the video card or maybe replace the card with a 5770?

atiflash is a DOS based flashing tool, cant be used in widows at all, you need to make a usb stick bootable and add the dos files and ati flash files and bios file. then you can force it to flash, but if you disable in bios the memory remapping/sharing it may take care of your problem